A new engine concept developed by researchers at the University of Wisconsin-Madison could cut fuel consumption by about 30 percent in cars and by almost 20 percent in heavy trucks. In gas-powered cars, the new design would add little to the cost of the engine. In heavy-duty trucks, it would substantially reduce costs by eliminating the need for expensive after-treatment systems to reduce emissions.
The concept, which has been demonstrated in test engines, involves precisely mixing two different fuels in the combustion chamber, which gives greater control over both the timing and duration of combustion. It could provide a way to meet fuel economy regulations without the more expensive electric motors and batteries found in hybrid vehicles (although for still greater efficiency, the new design could be incorporated into a hybrid vehicle). The new engine concept is made possible by precise electronic fuel injection and advances in computer simulations. “We discovered this process using advanced computer modeling, which allowed us to identify the recipe for optimal mixing of the fuels,” says Rolf Reitz, a professor of mechanical engineering at UW-Madison.
The design has two versions, one for replacing heavy-duty diesel engines and another, to be unveiled this fall, that would replace conventional gasoline engines. Both use the same combustion process that makes diesel engines significantly more efficient than gasoline engines–compressing fuel and air until it reaches pressures and temperatures that cause it to ignite, rather than using a spark to ignite the fuel. The new design improves engine efficiency beyond that of diesel engines by reducing the amount of energy wasted as heat and by improving control over the timing of combustion. It also greatly reduces the emissions associated with diesel engines, particularly important now that new emissions regulations require automakers to employ expensive after-treatment systems.
In the version designed to replace heavy-duty diesel engines, gasoline from one fuel tank is injected into the intake port near the combustion chamber, where it mixes with air before moving into the chamber (this is the conventional form of fuel injection in gasoline vehicles). Then diesel fuel from another tank is injected directly into the chamber using a low-pressure fuel injector. As this mixture is compressed, the diesel ignites first, followed shortly by the gasoline, which is more resistant to combustion. Controlling the ratio of the two fuels determines both the timing of the combustion and how long it lasts. The design requires precise control over the fuel injection, as the ratio and distribution of the two fuels in the chamber needs to change depending on the load placed on the engine. With light loads, the mix is about 50-50, while heavier loads might need as little as 5 percent diesel. The resulting engine is about 55 percent efficient, compared to 40 to 45 percent efficient for conventional heavy-duty diesel engines. Emissions are low enough to eliminate the need for after-treatment systems for exhaust–systems that, in a heavy-duty truck, can cost as much as the engine itself.